This invention relates to an improved electric furnace apparatus for use in the high temperature melting of glass. Specifically, the invention provides a method and apparatus designed to accomplish the efficient joule heating of molten glass. The electric heating of molten glass conventionally provides for a molten glass receptacle in which raw glass batch is heated and melted. The receptacle is lined with refractory. Heating electrodes, usually of opposed polarity, are immersed in the molten glass. Most commonly, the electrodes project through the refractory side walls or the refractory lined bottom of the molten glass receptacle. Since the electrodes are of opposed polarity, an electrical firing pattern is devised to provide electrical current between sets of electrodes of opposed polarity thereby raising or maintaining the temperature of the molten glass by means of joule electrical heating.
Many different electrode arrangements have been utilized, such as those shown in U.S. Pat. Nos. 4,528,013, 3,757,020 and 3,392,237. The one common factor in all electrode arrangements in joule glass melting furnaces lies in the factor that the electrodes are of opposite polarity in order to effect the joule current through the molten glass. A common goal is to achieve uniform temperature gradients throughout the molten glass and eliminate unwanted seeds, stones and cords. The large variety of prior art electrode arrangements indicate that it is difficult to achieve these desired goals with consistent results.
Another common problem regularly addressed by the large number of differing prior art electric furnace melters is that the glass melt receptacle must be constructed of a durable refractory which is less conductive to electric current than the molten glass. In other words, expressed in terms of electrical resistivity, the effective electrical resistivity of the refractory must be sufficient relative to the resistivity of the molten glass at the operative temperature of the glass heating to avoid any appreciable short circuiting of the heating current through the refractory rather than through the molten glass. With increasing temperature and glass fluidity, common refractory used in most furnaces becomes increasingly susceptible to being dissolved and corroded. As the refractory dissolves, many of the corrosive products are swept into the molten glass bath. The dissolved refractory materials become part of the glass composition and in some cases may have a deleterious effect on glass quality. Also, it is obvious that corrosion of the refractory leads to unwanted shutdown time and expense in repairing the corroded surfaces.
Many attempts to reduce the rate of corrosion or eliminate corrosion of the refractory are evident. For instance, U.S. Pat. No. 4,491,951 discloses a method and apparatus for cooling the refractory of the glass melt receptacle to increase the electrical resistivity of the refractory, thereby reducing the tendency of the refractory to short circuit and corrode. U.S. Pat. No. 4,366,571 discloses the use of a protective metal liner constructed of platinum or molybdenum to protect the refractory surface. U.S. Pat. No. 4,618,962 also discloses the use of a refractory metal liner such as molybdenum. A common goal of the prior art liner patents is the recognized desirability of avoiding electrical current charging to the metal liner from the electrodes. For instance, U.S. Pat. No. 4,618,962 avoids the use of electrodes inserted through the refractory and molybdenum lining since the lining would short circuit. To avoid the use of electrodes, U.S. Pat. No. 4,618,962 patent positions resistance heaters between the refractory and the lining to provide heat to the glass melt. U.S. Pat. No. 4,366,571 avoids the insertion of electrodes through the metal lining and instead, provides a variety of electrode arrangements wherein the electrodes are through-the-batch and floor electrodes, leaving the metal lining to the walls of the glass melt receptable.
Other attempts to avoid the declining electrical resistivity problem of refractory at high melt temperatures have involved building the furnace from highly resistive refractories such as zircon or zirconia. However, most refractories appear to have inversely proportional properties of electrical resistivity and corrosion resistance at increasing temperatures. Zircon-type refractories, while having high resistivity to electrical charging, have been found to be incompatible with certain glasses, such as C-glasses and are prone to high rates of erosion from such molten glass compositions. And, the utilization of refractories of higher erosion resistance such as chromic oxide refractories has not been found to be practical in some cases because the chromic oxide refractories have an electrical resistivity appreciably less than the resistivity of the molten glass at furnace operating temperatures. As a result, the chromic oxide refractory short circuits and current flows through the refractory, heating the refractory and thereby wearing the refractory excessively and sloughing the refractory into the molten glass. Thus corrective attempts to cool the chromic oxide refractories to avoid the short circuiting are evident, such as those shown in U.S. Pat. No. 4,491,951.
The present invention provides for a method and apparatus which improves upon these past efforts and achieves the following objectives.
It is an object of the present invention to provide a controlled electrical firing circuit to produce more thermally homogeneous glass.
It is an object of the present invention to reduce the level of stoning and cords in the glass melt due to refractory dissolution.
It is another object of the present invention to provide higher temperatures in the melter to produce lower seed counts.
It is another object of the present invention to provide for the high temperature melting of highly resistant glasses without corroding and short circuiting the refractory.
It is another object of the present invention to provide for the use of an erosion resistant, electrically conductive refractory without sacrificing the flexibility to use and melt highly resistant glasses.